Vintage Mustang Forums banner

1 - 7 of 7 Posts

·
Registered
Joined
·
1,260 Posts
Discussion Starter #1
The following reply appeared a couple of days ago and I plugged in the numbers and came out with 651 (use .85 rhater than the .085 in the post). I have questions about this ,however, that will follow after the reply with the formula.

This is a formula for estimating the needs of the motor:

rpm x disp / 3456 x .085 = cfm

Or, something like 6000 x 351 / 3456 x .085 = 517.9 cfm

This would be a 351 cid motor with peak rpm of 6000 and a volumetric efficiency of 85%. Obviously, some motors are built to rev higher and have a higher VE. So adjust the equation to suit your particular setup.

I prefer the Edelbrock for the street. I'm well aware that Chrissy Edelbrock runs a Holley in her race prepped Shelby. But I don't drive a race car. For the street an Edelbrock will keep up with a Holley just fine.


OK, The 650 seems just right at 8000 RPM. My build is:

331 (mexican block)
forged goodies with 10.5:1 compression
Aussie heads slightly ported with 2.05 intakes and 1.73 exhaust
AIR GAP RPM
294/300 roller cam with 544 lift (512 with 1.7 roller rockers)
balanced and blueprinted
3:55 rear- end
C4
2400 stall converter
650 speed demon (electric choke, vacuum advance and kickdown)
Powertrain 1 and 7/8 inch long tube headers

The kicker here is "how do I figure out my volumetric efficiency"?

I didn't want to overcarb. My builder says I will be good for 8000 RPM "at least" so I don't want to strangle out at 7000 (unless the dyno says don't go above there).

I'm going to ask my builder the same question when I get back in town, but I want you guys/(ok girls too!) to give me the benefit of your wisdom.
 

·
Registered
Joined
·
10,587 Posts
294/300 roller cam with 544 lift (512 with 1.7 roller rockers)

Please explain...generally, when using a 1.7 rocker, the lift is higher than with 1.6..

Car weight?

My instinct is this combo is going to need more converter and more gear...and suspension mods to match.....depends on intended use, hence....

Use? That will be telling... both for the type of carb as well as size. I know you said street but street with what performance bent..?


Generally speaking, I think a 650 would be great, with vacuum secondaries, but I would be leaning towards a Demon carb for easy adjustability with Edelbrock reliability...depends on how much you're wanting to tweek the combo...if the car was leaning more towards racing, you might also consider the 750..

Get back to us....
 

·
Registered
Joined
·
1,260 Posts
Discussion Starter #4
Pat,
Sorry I confused on the lift. The 544 comes from the 512 lift with 1.6 rockers and I'm running 1.7's, thus 512 divided by 1.6 X 1.7 = 544. Also it is a 650 Speed Demon!

This is a GT/CS 68 California Special. I don't know the exact weight as the only difference in this and a regular coupe is the little bit of fiberglass on the rear (decklid, end caps and tailight panel). I'll be running a 4 core with electric fan. I'm probably close at guessing 2800 lbs. (has dics brakes,A/C and PS).

The intended use is wild weekend street toy with occassional passes at the strip. I have installed KYB's and 1" front and 7/8" rear sway bars. The original springs seem to be in good condition, but I can change those out if need be.

I know desktop dyno is not to be used as the gospel, but it showed 330 ft. lbs. of torque at 2000 RPM with something like 164 HP, so I thought I would be OK with the 2400 stall. Do you think I should go with a 3000? I can still do that easily for a few bucks.

I agree wholeheartedly on the gear. I'll probably break the 8" soon and go with a 9" with lower gearing, but I've got to get SWMBO over the sticker shock of this first. TKO would also be nice, huh?

Thanks for your help. I always enjoy reading your replys.
 

·
Registered
Joined
·
10,587 Posts
Sorry I confused on the lift.

Not a problem...with the heat today, I'm just generally confused...*G*

Since acceleration from low speeds is on your mind (from your comments about the type of occasional track time the car will see), I'd like to see a converter similar to the one I use in the race car (10", 3200 rpm flash with the W) and a 3.70 or 4.11 gear (depending on highway use)...you can run a lower numerical gear, like you have now, with little penalty but IMO, the combo will optimize in the higher rpm ranges and gear change drops aren't as bad with the higher numerical rear gear.

Of course, you'll need some traction devices, good shocks (front and back) and maybe a pinion snubber for the track. The 8" should be OK with street tires and a foot brake...likely would OK for small slicks as well (8x26) if built right. It might be interesting to run the car across the scale at the track and weigh the axles...you might be surprised at both the weight and the distribution. Once known, steps can be taken to possibly re-distribute some weight and tune the suspension and chassis to transfer weight more efficiently.

The smaller carb with vacuum secondaries will help driveability without much sacrifice in the top end....I would only recommend a larger carb if this combo was running slicks and a 4.62 or higher numerical rear gear.

As an example, with an 26" slick and 4.11 gears, you'll be turning only about 5900 rpm @ 110 mph (allowing for tire growth and converter slip)....I'd like to see the combo, with your car weight, up around 6500 rpm or so for that speed. You can run low 12's at 110 or so...

As a point of comparison, your engine is substantially more radical the engines I ran 11.20's with in the race car....the difference is that those engines were optimized for racing and were allowed to reach their maximum performance levels without compromises for street operation (eg a lot of gear, 8" 5K stall converter, high C/R) as well as running in a lighter, more efficient vehicle (about 600 lbs lighter than the weight you computed). IME, these things make all the difference in the world....I went from a 14 second street car to an 11 second race car and did it with basically the same engine turning basically the same rpm....tuning the entire package and making it efficient are the keys, IMO.

Keep us updated on your progress!
 

·
Registered
Joined
·
1,260 Posts
Discussion Starter #6
Thanks Pat.

I will have 26x8.5 slicks for the track and subframe connectors. I will update when I get this bad boy in the car (2-3 weeks). For the purist reading this, I will be pulling the original engine and prepping for storage on a skid. None of the parts will be transferred to the new engine (lots of chrome and powder coat!!).

I'll get a higher rev converter for sure now!
 

·
Registered
Joined
·
3,848 Posts
Where'd that zero come from???? Sorry for that. You're correct. Should be 0.85.

Basically, volumetric efficiency is the capacity to fill the cylinder. Theoretically, if you have a 302 it would draw in 302 cubic inches of air on every intake cycle. That would be 100% VE. But since valve size, lift, carb, exhaust and head design all dictate the efficiency, not all engines operate at 100%. A good street engine is 75-85%. Some race prepped engines are capable of 100%+.

If you are using DTD 2000, it has a column for estimated VE. With the setup you're running, I would suspect you can achieve somewhere between 90-95% at peak.
 
1 - 7 of 7 Posts
Top